High Harmonic Generation Radiation Source

ABSTRACT

Methods and corresponding apparatus operable to cause an interaction between a drive radiation beam and a medium for generation of emitted radiation by high harmonic generation, the arrangement comprising: an interaction region positioned at an interaction plane and configured to receive the medium; a beam block positioned upstream of the interaction plane at a beam block plane and configured to partially block the drive radiation beam; a beam shaper positioned upstream of the beam block plane at an object plane and configured to control a spatial distribution of the drive radiation beam; and at least one lens positioned upstream of the interaction plane and downstream of the beam block plane, wherein the lens being positioned such that an image of the spatial distribution of the drive radiation beam is formed at the interaction plane.

FIELD

The invention relates to methods and apparatus for implementing aradiation source for generation of radiation using High HarmonicGeneration (HHG). More specifically, the invention may relate to methodsand apparatus for controlling an amount of a drive radiation thatescapes the radiation source. The invention may also relate to methodsand apparatus for inspection (e.g., metrology) usable, for example, inthe manufacture of devices by lithographic techniques using/includingsuch a radiation source.

BACKGROUND

A lithographic apparatus is a machine constructed to apply a desiredpattern onto a substrate. A lithographic apparatus can be used, forexample, in the manufacture of integrated circuits (ICs). A lithographicapparatus may, for example, project a pattern (also often referred to as“design layout” or “design”) at a patterning device (e.g., a mask) ontoa layer of radiation-sensitive material (resist) provided on a substrate(e.g., a wafer).

To project a pattern on a substrate a lithographic apparatus may useelectromagnetic radiation. The wavelength of this radiation determinesthe minimum size of features which can be formed on the substrate.Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nmand 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet(EUV) radiation, having a wavelength within the range 4-20 nm, forexample 6.7 nm or 13.5 nm, may be used to form smaller features on asubstrate than a lithographic apparatus which uses, for example,radiation with a wavelength of 193 nm.

Low-k₁ lithography may be used to process features with dimensionssmaller than the classical resolution limit of a lithographic apparatus.In such process, the resolution formula may be expressed as CD=k₁×λ/NA,where λ is the wavelength of radiation employed, NA is the numericalaperture of the projection optics in the lithographic apparatus, CD isthe “critical dimension” (generally the smallest feature size printed,but in this case half-pitch) and k₁ is an empirical resolution factor.In general, the smaller k₁ the more difficult it becomes to reproducethe pattern on the substrate that resembles the shape and dimensionsplanned by a circuit designer in order to achieve particular electricalfunctionality and performance. To overcome these difficulties,sophisticated fine-tuning steps may be applied to the lithographicprojection apparatus and/or design layout. These include, for example,but not limited to, optimization of NA, customized illumination schemes,use of phase shifting patterning devices, various optimization of thedesign layout such as optical proximity correction (OPC, sometimes alsoreferred to as “optical and process correction”) in the design layout,or other methods generally defined as “resolution enhancementtechniques” (RET). Alternatively, tight control loops for controlling astability of the lithographic apparatus may be used to improvereproduction of the pattern at low k1.

As explained below, an inspection apparatus, which may also be referredto as a metrology tool, may be used to determine properties ofsubstrates and features fabricated on substrates, and in particular, howproperties of different substrates vary or how properties associatedwith different layers of the same substrate vary from layer to layer.Such inspection apparatus may expose the substrate and the associatedfeatures to radiation and capture scattered or diffracted radiation toform images permitting the determination of the properties of thesubstrates and/or the features. The radiation may comprise soft x-ray(SXR) and/or EUV radiation.

Soft x-ray (SXR) and/or EUV radiation has a wavelength extending roughlyfrom 0.1 nm to 100 nm. Applications of SXR and/or EUV include, but arenot limited to, current or near-future measurement tools for thesemiconductor industry, e.g. where visible light is starting to giveinsufficient spatial resolution for the continuously shrinking featuresizes.

SXR and/or EUV radiation may be generated using HHG, in which, forexample, an intense laser pulse of visible or infrared (IR) driveradiation interacts with a gaseous medium, leading to emission of SXRand/or EUV by the gas atoms due to their interaction with the driveradiation. The HHG-generated SXR and/or EUV light may then be focusedonto a target on the wafer by means of an optical column that transfersthe light from the HHG source to the target. The reflected light may bedetected and processed to infer properties of the target.

SUMMARY

For the applicability of an SXR and/or EUV metrology tool it isdesirable to focus the SXR and/or EUV beam into a very small spot on thecustomer wafer. This is because usually only very little real estate onthe wafer is available for printing metrology targets. For manyexemplary use cases, the SXR and/or EUV spot should be smaller than 5 μmin diameter. This is a significant challenge which requires, amongstother things, a well-behaved and well-focusable SXR and/or EUV beamgenerated by the HHG source. The focusability of an SXR and/or EUV beamis determined by properties including beam divergence, intensitydistribution of the emitted SXR and/or EUV, and beam aberrations, ormore generally, by the wavefront of the SXR and/or EUV beam.

However, the inventors have appreciated that the atomic HHG mechanism issuch that the SXR and/or EUV wavefront is for a large part determined byan intensity distribution of the drive radiation in the gas target.Therefore, the inventors have realised that control over andoptimization of the focusability of the SXR and/or EUV beam, whichincreases the ability to implement an SXR and/or EUV metrology tool,depends at least in part on the control one has over the intensitydistribution of the drive laser in the gas target.

Typically in existing HHG sources, the drive radiation is focused downto a spot in the target with Gaussian intensity distribution. Within thelimitations of a gaussian spot, one has some rough control of the SXRand/or EUV wavefront by optimizing the drive laser focal spot sizeand/or the target position with respect to the focal point. However, theinventors have appreciated that more detailed control of the SXR and/orEUV wavefront may be achieved if one is not limited to a gaussiandistribution but instead is able to tailor a custom intensitydistribution of the drive radiation. For example, a different intensitydistribution may yield superior SXR and/or EUV wavefront properties.Technologies do exist to manipulate the focal spot distribution of alaser. In particular, the laser beam may be manipulated by deformablemirrors and spatial light modulators (SLMs) upstream of a lens thatfocuses the beam to a spot.

Further, in an HHG radiation source, the SXR and/or EUV beam (emittedradiation beam) is emitted in the same direction as the drive laserbeam. The drive radiation needs to be separated from the SXR and/or EUVbeam to prevent it interfering with the measurements. Additionally, therelatively high power drive radiation needs to be blocked in some way toprevent it from entering the sensitive optical column or beingtransmitted to the sensitive customer wafer.

Typically in existing HHG sources, blocking of the drive laser is doneby means of a thin metal foil that is partially transmissive to SXRand/or EUV. However, this method is not applicable to high-power HHGsources because such a filter cannot withstand high drive radiationpower. An alternative method has been proposed (Peatross et al., Opt.Lett. 19, 942 (1994)) in which the central part of the drive laser beamis blocked by a beam block. The resulting annular beam still makes agenerally Gaussian central spot at the focal point in the target, thusnot affecting the generation of SXR and/or EUV significantly, but willevolve again into an annular beam downstream of the target. The drivelaser may then be blocked by an aperture configured to allow the emittedSXR and/or EUV beam to pass through, whereas the annular drive radiationbeam is blocked. This is explained later with reference to FIG. 5 b.

The inventors have realised that improved solutions to one or more ofthe problems specified herein or otherwise known by a skilled person aredesirable. In some arrangements, methods and apparatus may seek to solveor mitigate the above two problems simultaneously. In exemplaryarrangements, problems associated with the separation of the driveradiation and emitted radiation (SXR and/or EUV beam) may be solved ormitigated by applying beam manipulation techniques to control theintensity distribution of the drive radiation beam, for example using abeam shaper such as an SLM. Problems associated with blocking the driveradiation at the output of the radiation source may be solved ormitigated by blocking the central part on the laser beam.

However, the inventors have realised that solutions to both of the aboveproblems may, in general, interfere with each other. That is, laser beamdistribution that is prepared upstream by an SLM, for example, would bemodified by a beam block arranged to form an annular drive radiationbeam, leading to an intensity distribution at the target other than thedesired one. Conversely, the annular beam property that should resultfrom the beam block will be affected by the beam manipulations of theSLM, leading to leakage of drive radiation through the downstreamaperture. Exemplary optical setups are proposed herein that circumventthis problem by making use of the imaging properties of a lens system.

According to the invention in an aspect there is provided a radiationsource arrangement operable to cause an interaction between a driveradiation beam and a medium for generation of emitted radiation by highharmonic generation, the arrangement comprising: an interaction regionpositioned at an interaction plane and configured to receive the medium;a beam block positioned upstream of the interaction plane at a beamblock plane and configured to partially block the drive radiation beam;a beam shaper positioned upstream of the beam block plane at an objectplane and configured to control a spatial distribution of the driveradiation beam; and at least one lens positioned upstream of theinteraction plane and downstream of the beam block plane, wherein thelens is positioned such that an image of the spatial distribution of thedrive radiation beam is formed at the interaction plane.

Optionally, the lens is positioned such that the object plane and theinteraction plane are conjugate planes.

Optionally, the arrangement further comprises an aperture positioneddownstream of the interaction plane at an aperture plane and configuredto allow at least part of the emitted radiation to pass through and toblock at least part of the drive radiation beam.

Optionally, the aperture plane is positioned with respect to the beamblock plane and the lens such that an image of the beam block is formedat the aperture plane.

Optionally, the lens is positioned such that the beam block plane andthe aperture plane are conjugate planes.

Optionally, a dimension of the beam block in the beam block planerelative to a dimension of the drive radiation beam in the beam blockplane is such that the image of the beam block and the image of thespatial distribution of the drive radiation beam are decoupled.

Optionally, the dimension of the beam block in the beam block plane is30% or less of the dimension of the drive radiation beam in the beamblock plane.

Optionally, the beam block and the drive radiation beam havesubstantially circular cross sections in the beam block plane, andwherein the dimensions of the beam block and the drive radiation beamare diameters.

Optionally, a depth of focus of the image of the beam block does notoverlap the interaction plane.

Optionally, a centre of the depth of focus of the image of the beamblock is substantially coincident with the aperture plane.

Optionally, a circle of confusion associated with the depth of focus ofthe image of the beam block is larger than the image of the driveradiation beam at the interaction plane.

Optionally, the depth of focus of the image of the beam block has amaximum circle of confusion having a diameter of 35 μm or less.

Optionally, a depth of focus of the image of the spatial distribution ofthe drive radiation beam does not overlap the aperture plane.

Optionally, a centre of the depth of focus of the image of the intensitydistribution of the drive radiation beam is substantially coincidentwith the interaction plane.

Optionally, the depth of focus of the image of the beam block and/or thedepth of focus of the image of the spatial distribution of the driveradiation beam is determined by

Depth of focus=2cN(1+m)

where c is a maximum circle of confusion, N is a ratio of focal lengthof the lens to a diameter of the drive radiation beam at the lens and mis a magnification factor of the image of the beam block at the apertureplane.

Optionally, the beam shaper comprises a spatial light modulatorpositioned at the object plane.

Optionally, the arrangement further comprises a sensor configured todetect a spatial profile of the drive radiation beam at the interactionplane; and a feedback controller configured to feedback data relating tothe detected spatial profile of the drive radiation beam to the spatiallight modulator, wherein the spatial light modulator is configured tocontrol the spatial profile of the drive radiation beam based on thedata fed back.

According to the invention in an aspect there is provided a metrologyapparatus comprising the arrangement according to any above or disclosedherein.

According to the invention in an aspect there is provided a lithographiccell comprising a arrangement discussed above or described elsewhereherein, or a metrology apparatus mentioned above or described elsewhereherein.

According to the invention in an aspect there is provided a method ofcausing an interaction between a drive radiation beam and a medium, forgeneration of emitted radiation by high harmonic generation, the methodcomprising: blocking the drive radiation beam by a beam block such thatat least part of the drive radiation beam is blocked; propagating thedrive radiation beam through at least one lens positioned downstream ofthe beam block; impinging the drive radiation beam on an interactionregion comprising the medium and positioned downstream of the beamblock; and focussing the drive radiation at an aperture positioneddownstream of the interaction region at an aperture plane such that animage of the beam block is formed at the aperture plane, the aperturebeing configured to allow at least part of the emitted radiation to passthrough and to block at least part of the drive radiation beam.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings, in which:

FIG. 1 depicts a schematic overview of a lithographic apparatus;

FIG. 2 depicts a schematic overview of a lithographic cell;

FIG. 3 depicts a schematic representation of holistic lithography,representing a cooperation between three key technologies to optimizesemiconductor manufacturing;

FIG. 4 depicts a schematic representation of a metrology apparatus;

FIG. 5a depicts a schematic representation of an HHG radiation source;

FIG. 5b depicts a schematic representation of an arrangement for usewith an HHG radiation source, including a beam block;

FIG. 6a depicts a schematic representation of an arrangement for usewith an HHG radiation source;

FIG. 6b depicts a schematic representation of an arrangement for usewith an HHG radiation source identifying exemplary distances between abeam block plane, a lens and an aperture plane;

FIG. 6c depicts a schematic representation of an arrangement for usewith an HHG radiation source identifying exemplary distances between anobject plane, a lens and an interaction plane;

FIG. 7 depicts a schematic representation of an arrangement for use withan HHG radiation source and illustrating a depth of field of a beamblock image;

FIG. 8 depicts a schematic representation of an arrangement for use withan HHG radiation source including a beam shaper;

FIG. 9 depicts a schematic representation of an arrangement for use withan HHG radiation source comprising two lenses; and

FIG. 10 shows a flow diagram for a method of causing an interactionbetween a drive radiation beam and a medium, for generation of emittedradiation by high harmonic generation.

DETAILED DESCRIPTION

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) andEUV (extreme ultra-violet radiation, e.g. having a wavelength in therange of about 5-100 nm).

The term “reticle”, “mask” or “patterning device” as employed in thistext may be broadly interpreted as referring to a generic patterningdevice that can be used to endow an incoming radiation beam with apatterned cross-section, corresponding to a pattern that is to becreated in a target portion of the substrate. The term “light valve” canalso be used in this context. Besides the classic mask (transmissive orreflective, binary, phase-shifting, hybrid, etc.), examples of othersuch patterning devices include a programmable mirror array and aprogrammable LCD array.

FIG. 1 schematically depicts a lithographic apparatus LA. Thelithographic apparatus LA includes an illumination system (also referredto as illuminator) IL configured to condition a radiation beam B (e.g.,UV radiation, DUV radiation or EUV radiation), a mask support (e.g., amask table) MT constructed to support a patterning device (e.g., a mask)MA and connected to a first positioner PM configured to accuratelyposition the patterning device MA in accordance with certain parameters,a substrate support (e.g., a wafer table) WT constructed to hold asubstrate (e.g., a resist coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate support inaccordance with certain parameters, and a projection system (e.g., arefractive projection lens system) PS configured to project a patternimparted to the radiation beam B by patterning device MA onto a targetportion C (e.g., comprising one or more dies) of the substrate W.

In operation, the illumination system IL receives a radiation beam froma radiation source SO, e.g. via a beam delivery system BD. Theillumination system IL may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic,electrostatic, and/or other types of optical components, or anycombination thereof, for directing, shaping, and/or controllingradiation. The illuminator IL may be used to condition the radiationbeam B to have a desired spatial and angular intensity distribution inits cross section at a plane of the patterning device MA.

The term “projection system” PS used herein should be broadlyinterpreted as encompassing various types of projection system,including refractive, reflective, catadioptric, anamorphic, magnetic,electromagnetic and/or electrostatic optical systems, or any combinationthereof, as appropriate for the exposure radiation being used, and/orfor other factors such as the use of an immersion liquid or the use of avacuum. Any use of the term “projection lens” herein may be consideredas synonymous with the more general term “projection system” PS.

The lithographic apparatus LA may be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system PS and the substrate W—which is also referred to asimmersion lithography. More information on immersion techniques is givenin U.S. Pat. No. 6,952,253, which is incorporated herein by reference.

The lithographic apparatus LA may also be of a type having two or moresubstrate supports WT (also named “dual stage”). In such “multiplestage” machine, the substrate supports WT may be used in parallel,and/or steps in preparation of a subsequent exposure of the substrate Wmay be carried out on the substrate W located on one of the substratesupport WT while another substrate W on the other substrate support WTis being used for exposing a pattern on the other substrate W.

In addition to the substrate support WT, the lithographic apparatus LAmay comprise a measurement stage. The measurement stage is arranged tohold a sensor and/or a cleaning device. The sensor may be arranged tomeasure a property of the projection system PS or a property of theradiation beam B. The measurement stage may hold multiple sensors. Thecleaning device may be arranged to clean part of the lithographicapparatus, for example a part of the projection system PS or a part of asystem that provides the immersion liquid. The measurement stage maymove beneath the projection system PS when the substrate support WT isaway from the projection system PS.

In operation, the radiation beam B is incident on the patterning device,e.g. mask, MA which is held on the mask support MT, and is patterned bythe pattern (design layout) present on patterning device MA. Havingtraversed the mask MA, the radiation beam B passes through theprojection system PS, which focuses the beam onto a target portion C ofthe substrate W. With the aid of the second positioner PW and a positionmeasurement system IF, the substrate support WT can be moved accurately,e.g., so as to position different target portions C in the path of theradiation beam B at a focused and aligned position. Similarly, the firstpositioner PM and possibly another position sensor (which is notexplicitly depicted in FIG. 1) may be used to accurately position thepatterning device MA with respect to the path of the radiation beam B.Patterning device MA and substrate W may be aligned using mask alignmentmarks M1, M2 and substrate alignment marks P1, P2. Although thesubstrate alignment marks P1, P2 as illustrated occupy dedicated targetportions, they may be located in spaces between target portions.Substrate alignment marks P1, P2 are known as scribe-lane alignmentmarks when these are located between the target portions C.

As shown in FIG. 2 the lithographic apparatus LA may form part of alithographic cell LC, also sometimes referred to as a lithocell or(litho)cluster, which often also includes apparatus to perform pre- andpost-exposure processes on a substrate W. Conventionally these includespin coaters SC to deposit resist layers, developers DE to developexposed resist, chill plates CH and bake plates BK, e.g. forconditioning the temperature of substrates W e.g. for conditioningsolvents in the resist layers. A substrate handler, or robot, RO picksup substrates W from input/output ports I/O1, I/O2, moves them betweenthe different process apparatus and delivers the substrates W to theloading bay LB of the lithographic apparatus LA. The devices in thelithocell, which are often also collectively referred to as the track,are typically under the control of a track control unit TCU that initself may be controlled by a supervisory control system SCS, which mayalso control the lithographic apparatus LA, e.g. via lithography controlunit LACU.

In order for the substrates W exposed by the lithographic apparatus LAto be exposed correctly and consistently, it is desirable to inspectsubstrates to measure properties of patterned structures, such asoverlay errors between subsequent layers, line thicknesses, criticaldimensions (CD), etc. For this purpose, inspection tools (not shown) maybe included in the lithocell LC. If errors are detected, adjustments,for example, may be made to exposures of subsequent substrates or toother processing steps that are to be performed on the substrates W,especially if the inspection is done before other substrates W of thesame batch or lot are still to be exposed or processed.

An inspection apparatus, which may also be referred to as a metrologyapparatus, is used to determine properties of the substrates W, and inparticular, how properties of different substrates W vary or howproperties associated with different layers of the same substrate W varyfrom layer to layer. The inspection apparatus may alternatively beconstructed to identify defects on the substrate W and may, for example,be part of the lithocell LC, or may be integrated into the lithographicapparatus LA, or may even be a stand-alone device. The inspectionapparatus may measure the properties on a latent image (image in aresist layer after the exposure), or on a semi-latent image (image in aresist layer after a post-exposure bake step PEB), or on a developedresist image (in which the exposed or unexposed parts of the resist havebeen removed), or even on an etched image (after a pattern transfer stepsuch as etching).

Typically the patterning process in a lithographic apparatus LA is oneof the most critical steps in the processing which requires highaccuracy of dimensioning and placement of structures on the substrate W.To ensure this high accuracy, three systems may be combined in a socalled “holistic” control environment as schematically depicted in FIG.3. One of these systems is the lithographic apparatus LA which is(virtually) connected to a metrology tool MT (a second system) and to acomputer system CL (a third system). The key of such “holistic”environment is to optimize the cooperation between these three systemsto enhance the overall process window and provide tight control loops toensure that the patterning performed by the lithographic apparatus LAstays within a process window. The process window defines a range ofprocess parameters (e.g. dose, focus, overlay) within which a specificmanufacturing process yields a defined result (e.g. a functionalsemiconductor device)—typically within which the process parameters inthe lithographic process or patterning process are allowed to vary.

The computer system CL may use (part of) the design layout to bepatterned to predict which resolution enhancement techniques to use andto perform computational lithography simulations and calculations todetermine which mask layout and lithographic apparatus settings achievethe largest overall process window of the patterning process (depictedin FIG. 3 by the double arrow in the first scale SC1). Typically, theresolution enhancement techniques are arranged to match the patterningpossibilities of the lithographic apparatus LA. The computer system CLmay also be used to detect where within the process window thelithographic apparatus LA is currently operating (e.g. using input fromthe metrology tool MT) to predict whether defects may be present due toe.g. sub-optimal processing (depicted in FIG. 3 by the arrow pointing“0” in the second scale SC2).

The metrology tool MT may provide input to the computer system CL toenable accurate simulations and predictions, and may provide feedback tothe lithographic apparatus LA to identify possible drifts, e.g. in acalibration status of the lithographic apparatus LA (depicted in FIG. 3by the multiple arrows in the third scale SC3).

As an alternative to optical metrology methods, it has also beenconsidered to use soft X-rays or EUV radiation, for example radiation ina wavelength range between 0.1 nm and 100 nm, or optionally between 1 nmand 50 nm or optionally between 10 nm and 20 nm. One example ofmetrology tool functioning in one of the above presented wavelengthranges is transmissive small angle X-ray scattering (T-SAXS as in US2007224518A which content is incorporated herein by reference in itsentirety). Profile (CD) measurements using T-SAXS are discussed byLemaillet et al in “Intercomparison between optical and X-rayscatterometry measurements of FinFET structures”, Proc. of SPIE, 2013,8681. Reflectometry techniques using X-rays (GI-XRS) and extremeultraviolet (EUV) radiation at grazing incidence are known for measuringproperties of films and stacks of layers on a substrate. Within thegeneral field of reflectometry, goniometric and/or spectroscopictechniques can be applied. In goniometry, the variation of a reflectedbeam with different incidence angles is measured. Spectroscopicreflectometry, on the other hand, measures the spectrum of wavelengthsreflected at a given angle (using broadband radiation). For example, EUVreflectometry has been used for inspection of mask blanks, prior tomanufacture of reticles (patterning devices) for use in EUV lithography.

It is possible that the range of application makes the use ofwavelengths in the soft X-rays or EUV domain not sufficient. Thereforepublished patent applications US 20130304424A1 and US2014019097A1(Bakeman et al/KLA) describe hybrid metrology techniques in whichmeasurements made using x-rays and optical measurements with wavelengthsin the range 120 nm and 2000 nm are combined together to obtain ameasurement of a parameter such as CD. A CD measurement may be obtainedby using an x-ray mathematical model and/or an optical mathematicalmodel. The contents of the cited US patent applications are incorporatedherein by reference in their entirety.

FIG. 4 depicts a schematic representation of a metrology apparatus 302in which radiation in the wavelength range from 0.1 nm to 100 nm may beused to measure parameters of structures on a substrate. The metrologyapparatus 302 presented in FIG. 4 is suitable for the soft X-rays or EUVdomain.

FIG. 4 illustrates a schematic physical arrangement of a metrologyapparatus 302 comprising a spectroscopic scatterometer using EUV and/orSXR radiation in grazing incidence, purely by way of example. Analternative form of inspection apparatus might be provided in the formof an angle-resolved scatterometer, which uses radiation in normal ornear-normal incidence similar to the conventional scatterometersoperating at longer wavelengths.

Inspection apparatus 302 comprises a radiation source 310, illuminationsystem 312, substrate support 316, detection systems 318, 398 andmetrology processing unit (MPU) 320.

Source 310 in this example comprises a generator of EUV or soft x-rayradiation based on high harmonic generation (HHG) techniques. Maincomponents of the radiation source are a drive laser 330, for producingdrive radiation, and an HHG gas cell 332. A gas supply 334 suppliessuitable gas target (or medium) to the gas cell, where it is optionallyionized by an electric source 336. The drive laser 300 may be, forexample, a fiber-based laser with an optical amplifier, producing pulsesof infrared radiation that may last for example less than 1 ns (1nanosecond) per pulse, with a pulse repetition rate up to severalmegahertz, as required. The wavelength of the infrared drive radiationmay be for example in the region of 1 μm (1 micron). The laser pulsesare delivered as a first radiation beam 340 to the HHG gas cell 332,where in the gas a portion of the radiation is converted to higherfrequencies than the first radiation into a beam 342 including coherentsecond (or emitted) radiation of the desired wavelength or wavelengths.

The second radiation may contain multiple wavelengths. If the radiationwere monochromatic, then measurement calculations (for examplereconstruction) may be simplified, but it is easier with HHG to produceradiation with several wavelengths. The volume of gas within the gascell 332 defines an HHG space, although the space need not be completelyenclosed and a flow of gas may be used instead of a static volume. Thegas may be for example a noble gas such as neon (Ne) or argon (Ar). N2,O2, He, Ar, Kr, Xe gases can all be considered. These are matters ofdesign choice, and may even be selectable options within the sameapparatus. Different wavelengths will, for example, provide differentlevels of contrast when imaging structure of different materials. Forinspection of metal structures or silicon structures, for example,different wavelengths may be selected to those used for imaging featuresof (carbon-based) resist, or for detecting contamination of suchdifferent materials. One or more filtering devices 344 may be provided.For example a filter such as a thin membrane of Aluminum (Al) may serveto cut the fundamental IR radiation from passing further into theinspection apparatus. A grating (not shown) may be provided to selectone or more specific harmonic wavelengths from among those generated inthe gas cell. Some or all of the beam path may be contained within avacuum environment, bearing in mind that SXR and/or EUV radiation isabsorbed when traveling in air. The various components of radiationsource 310 and illumination optics 312 can be adjustable to implementdifferent metrology ‘recipes’ within the same apparatus. For exampledifferent wavelengths and/or polarization can be made selectable.

Depending on the materials of the structure under inspection, differentwavelengths may offer a desired level of penetration into lower layers.For resolving the smallest device features and defects among thesmallest device features, then a short wavelength is likely to bepreferred. For example, one or more wavelengths in the range 1-20 nm oroptionally in the range 1-10 nm or optionally in the range 10-20 nm maybe chosen. Wavelengths shorter than 5 nm suffer from very low criticalangle when reflecting off materials typically of interest insemiconductor manufacture. Therefore to choose a wavelength greater than5 nm will provide stronger signals at higher angles of incidence. On theother hand, if the inspection task is for detecting the presence of acertain material, for example to detect contamination, then wavelengthsup to 50 nm could be useful.

From the radiation source 310, the filtered beam 342 enters aninspection chamber 350 where the substrate W including a structure ofinterest is held for inspection at a measurement position by substratesupport 316. The structure of interest is labeled T. The atmospherewithin inspection chamber 350 is maintained near vacuum by vacuum pump352, so that EUV radiation can pass with-out undue attenuation throughthe atmosphere. The Illumination system 312 has the function of focusingthe radiation into a focused beam 356, and may comprise for example atwo-dimensionally curved mirror, or a series of one-dimensionally curvedmirrors, as described in published US patent applicationUS2017/0184981A1 (which content is incorporated herein by reference inits entirety), mentioned above. The focusing is performed to achieve around or elliptical spot S under 10 Lm in diameter, when projected ontothe structure of interest. Substrate support 316 comprises for examplean X-Y translation stage and a rotation stage, by which any part of thesubstrate W can be brought to the focal point of beam to in a desiredorientation. Thus the radiation spot S is formed on the structure ofinterest. Alternatively, or additionally, substrate support 316comprises for example a tilting stage that may tilt the substrate W at acertain angle to control the angle of incidence of the focused beam onthe structure of interest T.

Optionally, the illumination system 312 provides a reference beam ofradiation to a reference detector 314 which may be configured to measurea spectrum and/or intensities of different wavelengths in the filteredbeam 342. The reference detector 314 may be configured to generate asignal 315 that is provided to processor 310 and the filter may compriseinformation about the spectrum of the filtered beam 342 and/or theintensities of the different wavelengths in the filtered beam.

Reflected radiation 360 is captured by detector 318 and a spectrum isprovided to processor 320 for use in calculating a property of thetarget structure T. The illumination system 312 and detection system 318thus form an inspection apparatus. This inspection apparatus maycomprise a soft X-ray and/or EUV spectroscopic reflectometer of the kinddescribed in US2016282282A1 which content is incorporated herein byreference in its entirety.

If the target T has a certain periodicity, the radiation of the focussedbeam 356 may be partially diffracted as well. The diffracted radiation397 follows another path at well-defined angles with respect to theangle of incidence then the reflected radiation 360. In FIG. 4, thedrawn diffracted radiation 397 is drawn in a schematic manner anddiffracted radiation 397 may follow many other paths than the drawnpaths. The inspection apparatus 302 may also comprise further detectionsystems 398 that detect and/or image at least a portion of thediffracted radiation 397. In FIG. 4 a single further detection system398 is drawn, but embodiments of the inspection apparatus 302 may alsocomprise more than one further detection system 398 that are arranged atdifferent position to detect and/or image diffracted radiation 397 at aplurality of diffraction directions. In other words, the (higher)diffraction orders of the focussed radiation beam that impinges on thetarget T are detected and/or imaged by one or more further detectionsystems 398. The one or more detection systems 398 generates a signal399 that is provided to the metrology processor 320. The signal 399 mayinclude information of the diffracted light 397 and/or may includeimages obtained from the diffracted light 397.

To aid the alignment and focusing of the spot S with desired productstructures, inspection apparatus 302 may also provide auxiliary opticsusing auxiliary radiation under control of metrology processor 320.Metrology processor 320 can also communicate with a position controller372 which operates the translation stage, rotation and/or tiltingstages. Processor 320 receives highly accurate feedback on the positionand orientation of the substrate, via sensors. Sensors 374 may includeinterferometers, for example, which can give accuracy in the region ofpicometers. In the operation of the inspection apparatus 302, spectrumdata 382 captured by detection system 318 is delivered to metrologyprocessing unit 320.

As mentioned an alternative form of inspection apparatus uses soft X-rayand/or EUV radiation at normal incidence or near-normal incidence, forexample to perform diffraction-based measurements of asymmetry. Bothtypes of inspection apparatus could be provided in a hybrid metrologysystem. Performance parameters to be measured can include overlay (OVL),critical dimension (CD), coherent diffraction imaging (CDI) andat-resolution overlay (ARO) metrology. The soft X-ray and/or EUVradiation may for example have wavelengths less than 100 nm, for exampleusing radiation in the range 5-30 nm, of optionally in the range from 10nm to 20 nm. The radiation may be narrowband or broadband in character.The radiation may have discrete peaks in a specific wavelength band ormay have a more continuous character.

Like the optical scatterometer used in today's production facilities,the inspection apparatus 302 can be used to measure structures withinthe resist material treated within the litho cell (After DevelopInspection or ADI), and/or to measure structures after they have beenformed in harder material (After Etch Inspection or AEI). For example,substrates may be inspected using the inspection apparatus 302 afterthey have been processed by a developing apparatus, etching apparatus,annealing apparatus and/or other apparatus.

FIG. 5a shows a schematic representation of a radiation source 500 forgeneration of emitted radiation by HHG. The source 500 comprises avacuum vessel 502 and a vacuum optical system 504 including the opticalcolumn referred to above and directing an emitted beam of radiation ontoa substrate. The vacuum vessel 502 comprises an interaction region 506for receiving a medium 508 for use as a target, such as a gas target.The vacuum vessel 502 comprises a viewport 510 or other inlet to thevacuum vessel 502 through which drive radiation 512 enters the vacuumvessel 502. At the interaction region 506, the drive radiation 512interacts with the medium 508 to generate emitted radiation 514 by HHG.It can be seen in FIG. 5 that the drive radiation 512 continues topropagate beyond the interaction region 506 in the direction of emissionof the emitted radiation 514. For reasons mentioned above, a blockingfilter 516 may be used to block at least part of the drive radiation 512and allow at least part of the emitted radiation 514 to pass. Typicallysuch a filter may attenuate the drive radiation by many orders ofmagnitude (e.g. only 10⁻¹¹ of the incident drive radiation may pass thefilter), and typically some tens of percents of the HHG radiation canpass (e.g. 20%).

FIG. 5b shows an exemplary arrangement for use with the source 500 andfor blocking drive radiation 512 to prevent it being emitted from thesource 500. The features of FIG. 5b may be combined as appropriate withthose of FIG. 5a . The arrangement of FIG. 5b comprises a beam block 518positioned upstream (i.e. closer to the drive radiation source) of theinteraction region 506. The beam block 518 blocks at least part of thedrive radiation and in the arrangement of FIG. 5b results in an annulardrive radiation beam. The term “annular” in this sense encompasses anyshape of beam with an internal area that is blocked. The beam block maybe a physical block placed in the propagation path of the driveradiation 512 or may be provided by other arrangements such as a piercedmirror or an axicon pair.

A lens 520 is positioned downstream of the beam block 518 and upstreamof the interaction region 506 and is arranged to focus the driveradiation 512 at the interaction region 506. Using such an arrangement,a roughly Gaussian intensity distribution of the drive radiation 512 isseen at the interaction region 506, after which the drive radiation 512continues to propagate as an annular beam downstream of the interactionregion 506. The blocking filter 516 comprises an aperture 522. Theaperture 522 is sized to permit a substantial amount of the driveradiation 512 to be blocked and to allow a substantial amount of theemitted radiation 514 to pass through. Typically, the aperture may beselected such that at least 99% or 99.9% of the drive radiation isblocked such that the remaining part is of sufficiently low intensitynot to damage any further filter that may be placed downstream.Additionally, it is desirable to limit loss of HHG radiation by theaperture so it may be selected to allow at least some tens of percent topass through (e.g. 20%).

FIG. 6a shows a schematic representation of an arrangement for use in aradiation source. The features of FIG. 6a may be combined with those ofFIG. 5a as appropriate. The arrangement of FIG. 6a comprises aninteraction region 606 that is positioned at an interaction plane and isconfigured to receive a medium, such as a gas target, for generation ofemitted radiation by HHG. The arrangement also comprises a beam block618 that is positioned upstream from the interaction region 606 at abeam block plane and that is positioned to block at least part of adrive radiation beam (not shown in FIG. 6a ), and in specificarrangements to block a central part of the drive radiation beam. Thearrangement also comprises a lens 620 that is positioned upstream of theinteraction region 606 and downstream of the beam block 618. Thearrangement also comprises an aperture 622 positioned downstream of theinteraction region 606 at an aperture plane. It is noted here thatwhilst the aperture plane is substantially perpendicular to thedirection of propagation of drive radiation through the source, theaperture 622 itself may be transverse to a direction of propagation ofdrive radiation through the source. The term “transverse” as used inthis context encompasses perpendicular to, but need not be limited tothat definition. In exemplary arrangements, the aperture 622 may beangled with respect to the aperture plane to reflect drive radiationaway from an axis of propagation of the drive radiation. For example,the aperture may be formed in a mirror that is angled with respect tothe axis of propagation. In such arrangements, at least a part of theaperture 622 will be coincident with the aperture plane. As with otherarrangements described herein, the aperture 622 is configured to allowat least part of the emitted radiation to pass through and to block atleast part of the drive radiation beam. typically an SXR and/or EUVefficient design would permit a significant fraction, say more than 50%,and ideally >90%, to pass through.

Drive laser blocking efficiency may depend on the sensitivity to thedrive radiation of the optics, sample and measurements downstream.Typically, this sensitivity is very high, and residual drive radiationpassing the block may be many orders smaller than the input beam.Therefore, typically a metallic filter may be used downstream of theblocking aperture to filter out any residual drive radiation passing theaperture. Assuming this situation, the blocking efficiency of theaperture may be set according to the damage threshold of the downstreamfilter. Typically, such a filter can tolerate no more than 1 W of laser,corresponding to a blocking efficiency of 99% or so.

Note that within this invention, forming an image means that an imageplane is positioned within the depth of focus (DOF) of a lens. Thedefinition of forming an image of a beam block is that the distancebetween the aperture plane and the conjugate plane of the beam blockplane is smaller than the depth of focus. The definition of forming animage of the spatial distribution of the drive radiation beam is thatthe distance between the interaction plane and the conjugate plane ofthe object plane is smaller than the depth of focus.

The definition of forming a sharp image is that the image plane is theconjugate plane.

In the arrangement of FIG. 6a , the aperture plane is positioned withrespect to the beam block plane and the lens 620 such that an image ofthe beam block is formed at the aperture plane. The beam block plane andthe aperture plane are set up such that they may be conjugate planes.This results in the drive radiation beam cross-section having awell-defined hole in the center when arriving at the aperture plane(i.e. the laser beam will be annular at the aperture plane), permittingeffective blocking of the drive laser beam by an aperture whiletransmitting the emitted (e.g. SXR and/or EUV) beam.

As can be seen in FIG. 6a , the arrangement also comprises an objectplane. The object plane may be the conjugate plane of the gas targetplane, that is, the lens forms a sharp image of the object plane in thegas target plane. In exemplary arrangements (e.g. shown in FIG. 6c ) adesired intensity distribution of the drive radiation is formed in theobject plane. In other exemplary arrangements (e.g. in FIG. 8) there maynot be a real plane where the desired distribution is created andtherefore the object plane may not be involved. In some exemplaryarrangements and as explained below, the object plane and theinteraction plane may also be positioned such that a sharp image of theintensity of the drive radiation at the object plane is formed at theinteraction plane, that is, they are conjugate planes.

FIG. 6b shows a schematic representation of the beam block 618, lens620, interaction region 606 and aperture 622. The relative distancesbetween the beam block plane and the lens 620, and between the lens 620and the aperture plane may be calculated using the equations

$d_{{block} - {lens}} = {\frac{1 + m_{b}}{m_{b}}f}$d_(lens − aperture) = (1 + m_(b))f

Where m_(b) is the magnification factor of the beam block to the imageof the beam block provided by the lens 620 and f is the focal length ofthe lens 620.

The drive radiation intensity distributions at both the beam block plane624 and the aperture plane 626 are shown in FIG. 6b and show that theintensity is zero in a central region of the drive radiation 626 at theaperture plane.

As mentioned above, in some exemplary arrangements, the object plane andthe interaction plane are also conjugate planes, resulting in the driveradiation having a desired intensity distribution at the object plane628 being imaged at the interaction plane 630. This is shown in FIG. 6cand the relative distances between the object plane and the lens 620,and between the lens 620 and the interaction plane may be calculatedusing the equations

$d_{{obj} - {lens}} = {\frac{1 + m_{s}}{m_{s}}f}$d_(lens − inter) = (1 + m_(s))f

Where m_(s) is the magnification factor of the object plane to the imageof the object plane provided by the lens 620 and f is the focal lengthof the lens 620.

Such arrangements can ensure that, in addition to the beam block planebeing imaged at the aperture plane, the object plane (having the correctintensity distribution of the drive radiation) is imaged at theinteraction plane. In some such exemplary arrangements, the image of theobject plane may be decoupled from the image of the beam block planesuch that they do not interfere with each other, or at least that theyinterfere with each other sufficiently little that operation of thesource is not adversely affected. The term “decoupled” in this contextmay encompass a situation where the presence of the beam block does notappreciably influence the intensity distribution of the drive radiationat the target plane and the presence of a non-Gaussian beam at theobject plane does not appreciably influence the intensity distributionof the drive radiation at the aperture plane.

In exemplary arrangements, the dimensions of the beam block 618 at thebeam block plane relative to the dimensions of the drive radiation atthe beam block plane may be configured to ensure that an image of thebeam block 618 does not appear at the interaction region. In someexemplary arrangements, the dimensions of the beam block 618 may be atleast 20% less, at least 40% less, at least 50% less or at least 70%less than the corresponding dimensions of the drive radiation beam.Further, in exemplary arrangements, the dimensions of the beam block 618may be from 10% to 70% less or 20% to 60% less or 30% to 50% less thanthe corresponding dimensions of the drive radiation beam.

In other exemplary arrangements, the dimensions of the beam block 618and the drive radiation at the beam block plane may be configured toachieve a Strehl ratio of 0.8 or more. In arrangements where the driveradiation beam and the beam block 618 has a circular cross section, thedimensions mentioned above may be diameters.

Decoupling may also be defined in terms of a depth of focus (DOF) of oneor both of the image of the beam block at the aperture plane and theimage of the drive radiation beam at the interaction plane. DOF in thiscontext may be defined as a range of distances from a lens at which aimage may be formed, which is explained in greater detail below.

For the purposes of the methods and apparatus disclosed herein, theinteraction plane may be positioned outside of the DOF of the beam blockimage. That is, the aperture plane, which in some exemplary arrangementsis coincident with a centre of the DOF of the beam block image, may besufficiently distanced from the interaction plane so that theinteraction plane falls outside the DOF of the beam block image. Noimage of the beam block is formed at the interaction plane.

Similarly, the aperture plane may be positioned such that it is outsideof the DOF of the image of the drive radiation intensity distributionformed at the interaction plane. That is, the interaction plane, whichin some exemplary arrangements is coincident with a centre of the DOF ofthe image of the drive radiation intensity distribution, may besufficiently distanced from the aperture plane so that the apertureplane falls outside the DOF of the image of the drive radiationintensity distribution. No image of the drive radiation intensitydistribution at the object plane is formed at the aperture plane.

FIG. 7 shows a schematic demonstrating the principle of DOF. As can beseen, rays of radiation emanating from the beam block 618 are focussedby the lens 620 to a point that is coincident with the aperture plane.This point is the point at which the image of the beam block 618 may bein sharp focus and may be the center of the DOF. At positions upstreamand downstream of the center of the DOF, the image of the beam block 618is out of focus to a degree that increases with distance away from thecenter of the DOF. At these positions away from the center of the DOF,the rays of radiation from the beam block 618 are distributed over ablurred spot, which is termed a circle of confusion.

The DOF of the beam block image is the distance either side of theaperture plane within which the circle of confusion is less than orequal to a maximum diameter, c. The diameter, c, may be set at a sizewhereby the beam block image is blurred all over the relevant radialdomain. The interaction plane may be positioned at a distance furtherfrom the aperture plane than would result in a circle of confusion ofdiameter, c, outside of the DOF. That is, at the gas target plane, thecircle of confusion should be larger than the size of the fielddistribution in the gas target plane, which is usually several tens ofum. Half the DOF should be smaller than the distance between the gastarget plane and aperture plane.

The DOF may be calculated using the following formula

DOF=2cN(1+m)

Where N is the ratio of focal length to beam diameter at the lens 620(typically around 100), and m is the magnification of the beam blockimage by the lens 620 (typically around 1). Using those typical figuresand assuming c=30 μm, the DOF is approximately 10 mm. The features ofthe arrangements disclosed herein may be configured such that the DOF ofthe beam block image is in a range from 7 mm to 13 mm, from 8 mm to 12mm and in specific examples from 10 mm to 11 mm. In some arrangements,the diameter, c, of the circle of confusion of the beam block image is35 μm or less or 30 μm or less.

Exemplary arrangements for use in a radiation source may comprise a beamshaper. The beam shaper may be any apparatus capable of controlling anintensity distribution of the drive radiation, which may be produced bya laser as discussed above. Examples of a beam shaper include a spatiallight modulator (SLM), a flattop beam shaper and a deformable mirror.The beam shaper may be placed significantly closer to the beam blockthan a real object plane, so that the overall system size may besignificantly reduced. The beam shaper may be used to generate the driveradiation beam having a desired intensity distribution at the objectplane. In other arrangements, the beam shaper may also provide a desiredintensity distribution at any other plane upstream of the beam block. Inexemplary arrangements using a beam shaper there may not be a realobject plane left of the lens in which the desired distribution at thetarget plane is reproduced and in such circumstances the object planemay be considered a virtual object plane.

FIG. 8 shows an exemplary arrangement for use with a radiation source inwhich a beam shaper 800 is positioned upstream of the beam block 618.The beam shaper 800 receives drive radiation 802 from a drive radiationsource (not shown) and having, for example, a broadly Gaussian intensitydistribution and generates drive radiation 804 having a specificintensity distribution. The arrangement also includes a sensor and afeedback controller 806. The sensor senses the intensity distribution ofthe drive radiation at the interaction plane and passes it to thefeedback controller 806. The feedback controller 806 sends data relatingto the sensed intensity distribution to the beam shaper 804, whichcontrols the intensity distribution it outputs based thereon. In anembodiment, a 1% beam splitter is used to project a portion of the driveradiation towards a CCD camera which acts as the above indicated sensor.The position of such a beam splitter may be, seen in the transmissiondirection of the drive radiation, just before or just after the lens.

The arrangement of FIG. 8 may include one or more features ofarrangements disclosed above and may be combined with as appropriatewith features of FIG. 5 a.

FIG. 9 shows an exemplary arrangement comprising two lenses 620 a, 620b. This allows the overall length of the system to be reduced. FIG. 9may also include one or more of the features disclosed in respect ofother arrangements above and/or may be combined as appropriate withfeatures of FIG. 5 a.

The two lens system of FIG. 9 provides no intermediate focus of thedrive radiation as this would lead to drive radiation beam deteriorationdue to ionization of air. The second lens 620 b is a negative lensforming a virtual intermediate image 900 rather than real intermediateimage of the intensity distribution of the drive radiation of the objectplane. The second lens 620 b is a negative lens. The second lens 620 bis upstream of the beam block 618.

The distance of the second lens 620 b from the object plane can bedetermined by the equation

$d_{{obj} - {{lens}\; 2}} = {\frac{1 - m_{-}}{m_{-}}{f_{-}}}$

Where m⁻ is the magnification of the image of the intensity of the driveradiation beam by the second lens 620 b and f⁻ is the focal length ofthe second lens 620 b. The distance of the second lens 620 b from thevirtual intermediate image 900 can be determined by the equation

d _(lens2-virtual)=(1−m ⁻)|f ⁻|

The distance of the first lens 620 a from the virtual intermediate image900 can be determined by the equation

$d_{{virtual} - {{lens}\; 1}} = {\frac{1 + m_{+}}{m_{+}}{f_{+}}}$

Where m₊ is the magnification of the virtual image 900 of the intensityof the drive radiation beam by the first lens 620 a and f₊ is the focallength of the first lens 620 a. The distance of the first lens 620 afrom the virtual intermediate image 900 can be determined by theequation

d _(lens1-inter)=(1+m ₊)f ₊

FIG. 10 shows a flow diagram of a method for causing an interactionbetween a drive radiation beam and a medium, for generation of emittedradiation by high harmonic generation.

Drive radiation generated by a drive radiation source such as a laserand is propagated 1000 into a vacuum vessel, such as that shown in FIG.5a . At least part of the drive radiation is blocked 1002 by a beamblock 618. The beam block 618 may be as described above or of any otherform known to the skilled person. The drive radiation that is partiallyblocked propagates through a lens 1004 that focusses it onto theinteraction plane. The focussed drive radiation impinges 1006 on amedium, such as a gas target that is positioned downstream of the beamblock and the lens. The lens also focusses drive radiation emanatingfrom the beam block 618 at the aperture plane positioned downstream ofthe interaction plane such that an image of the beam block 618 is formedat the aperture plane.

Further embodiments are defined in the subsequent numbered clauses:

1. A radiation source arrangement operable to cause an interactionbetween a drive radiation beam and a medium for generation of emittedradiation by high harmonic generation, the arrangement comprising:

-   -   an interaction region positioned at an interaction plane and        configured to receive the medium;

a beam block positioned upstream of the interaction plane at a beamblock plane and configured to partially block the drive radiation beam;and

a beam shaper positioned upstream of the beam block plane at an objectplane and configured to control a spatial distribution of the driveradiation beam.

2. The arrangement according to clause 1, wherein at least one lens ispositioned upstream of the interaction plane and downstream of the beamblock plane, wherein the lens is positioned such that an image of thespatial distribution of the drive radiation beam is formed at theinteraction plane.

3. The arrangement according to clause 1 or 2, wherein the lens ispositioned such that the object plane and the interaction plane areconjugate planes.

4. The arrangement according to any preceding clause, wherein anaperture is positioned downstream of the interaction plane at anaperture plane and configured to allow at least part of the emittedradiation to pass through and to block at least part of the driveradiation beam, wherein the aperture plane is positioned with respect tothe beam block plane and the lens such that an image of the beam blockis formed at the aperture plane.

5. The arrangement according to clause 4, wherein the lens is positionedsuch that the beam block plane and the aperture plane are conjugateplanes.

6. The arrangement according to clause 4 or 5, wherein a dimension ofthe beam block in the beam block plane relative to a dimension of thedrive radiation beam in the beam block plane is such that the image ofthe beam block and the image of the spatial distribution of the driveradiation beam are decoupled.

7. The arrangement according to clause 6, wherein the dimension of thebeam block in the beam block plane is 30% or less of the dimension ofthe drive radiation beam in the beam block plane.

8. The arrangement according to clause 7, wherein the beam block and thedrive radiation beam have substantially circular cross sections in thebeam block plane, and wherein the dimensions of the beam block and thedrive radiation beam are diameters.

9. The arrangement according to any preceding clause, wherein a depth offocus of the image of the beam block does not overlap the interactionplane.

10. The arrangement according to clause 9, wherein a centre of the depthof focus of the image of the beam block is substantially coincident withthe aperture plane.

11. The arrangement according to clause 9 or 10 when dependent directlyor indirectly on clause 4, wherein a circle of confusion associated withthe depth of focus of the image of the beam block is larger than theimage of the drive radiation beam at the interaction plane.

12. The arrangement according to clause 11, wherein the depth of focusof the image of the beam block has a maximum circle of confusion havinga diameter of 35 μm or less.

13. The arrangement according to any preceding clause, wherein a depthof focus of the image of the spatial distribution of the drive radiationbeam does not overlap the aperture plane.

14. The arrangement according to clause 13, wherein a centre of thedepth of focus of the image of the intensity distribution of the driveradiation beam is substantially coincident with the interaction plane.

15. The arrangement according to any of clauses 9 to 14, wherein thedepth of focus of the image of the beam block and/or the depth of focusof the image of the spatial distribution of the drive radiation beam isdetermined by

Depth of focus=2cN(1+m)

where c is a maximum circle of confusion, N is a ratio of focal lengthof the lens to a diameter of the drive radiation beam at the lens and mis a magnification factor of the image of the beam block at the apertureplane.

16. The arrangement according to any of clauses 4 to 15 when dependentdirectly or indirectly on clause 3, wherein the beam shaper comprises aspatial light modulator positioned at the object plane.

17. The arrangement of clause 16, further comprising a sensor configuredto detect a spatial profile of the drive radiation beam at theinteraction plane; and a feedback controller configured to feedback datarelating to the detected spatial profile of the drive radiation beam tothe spatial light modulator, wherein the spatial light modulator isconfigured to control the spatial profile of the drive radiation beambased on the data fed back.

18. A metrology apparatus comprising the arrangement according to anypreceding clause.

19. A lithographic cell comprising the arrangement according to anypreceding clause or the metrology apparatus according to clause 18.

20. A method of causing an interaction between a drive radiation beamand a medium, for generation of emitted radiation by high harmonicgeneration, the method comprising:

-   -   blocking the drive radiation beam by a beam block such that at        least part of the drive radiation beam is blocked;    -   propagating the drive radiation beam through at least one lens        positioned downstream of the beam block;

impinging the drive radiation beam on an interaction region comprisingthe medium and positioned downstream of the beam block; and

focussing the drive radiation at an aperture positioned downstream ofthe interaction region at an aperture plane such that an image of thebeam block is formed at the aperture plane, the aperture beingconfigured to allow at least part of the emitted radiation to passthrough and to block at least part of the drive radiation beam.

In the context of this document the term HHG or HHG source isintroduced. HHG refers to High Harmonic Generation, sometimes referredto as high order harmonic generation. HHG is a non-linear process inwhich a target, for example a gas, a plasma or a solid sample, isilluminated at an interaction region by an intensive laser pulse ofdrive radiation. Subsequently, the target may emit radiation with afrequency that is a multiple of the frequency of the drive radiation ofthe laser pulse. Such frequency, that is a multiple, is called aharmonic of the radiation of the laser pulse. One may define that thegenerated HHG radiation is a harmonic above the fifth harmonic and theseharmonics are termed high harmonics.

The physical process that forms a basis of the HHG process is differentfrom the physical process that relates to generating radiation of thelower harmonics, typically the 2^(nd) to 5^(th) harmonic. The generationof radiation of the lower harmonic relates to perturbation theory. Thetrajectory of the (bound) electron of an atom in the target issubstantially determined by the Coulomb potential of the host ion.

In HHG, the trajectory of the electron that contributes to the HHGprocess is substantially determined by the electric field of theincoming drive laser light. In the so-called “three step model”describing HHG, electrons tunnel through the Coulomb barrier which is atthat moment substantially suppressed by the laser field (step 1), followa trajectory determined by the laser field (step 2) and recombine with acertain probability while releasing their kinetic energy plus theionization energy in the form of radiation (step 3). Another way ofphrasing a difference between HHG and the generation of radiation of thelower harmonic is to define that all radiation with photon energy abovethe ionization energy of the target atoms as “High Harmonic” radiation,e.g. HHG generated radiation, and all radiation with photon energy belowthe ionization energy as non-HHG generated radiation. If Neon is used asa gas target, all radiation with a wavelength shorter than 62 nm (havinga photon energy higher than 20.18 eV) is generated by means of the HHGprocess. For Argon as a gas target, all radiation having a photon energyhigher than about 15.8 eV is generated by means of the HHG process.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications. Possible other applications include the manufactureof integrated optical systems, guidance and detection patterns formagnetic domain memories, flat-panel displays, liquid-crystal displays(LCDs), thin-film magnetic heads, etc.

Although specific reference may be made in this text to embodiments ofthe invention in the context of a lithographic apparatus, embodiments ofthe invention may be used in other apparatus. Embodiments of theinvention may form part of a mask inspection apparatus, a metrologyapparatus, or any apparatus that measures or processes an object such asa wafer (or other substrate) or mask (or other patterning device). Theseapparatus may be generally referred to as lithographic tools. Such alithographic tool may use vacuum conditions or ambient (non-vacuum)conditions.

Although specific reference is made to “metrology apparatus/tool/system”or “inspection apparatus/tool/system”, these terms may refer to the sameor similar types of tools, apparatuses or systems. E.g. the inspectionor metrology apparatus that comprises an embodiment of the invention maybe used to determine characteristics of structures on a substrate or ona wafer. E.g. the inspection apparatus or metrology apparatus thatcomprises an embodiment of the invention may be used to detect defectsof a substrate or defects of structures on a substrate or on a wafer. Insuch an embodiment, a characteristic of interest of the structure on thesubstrate may relate to defects in the structure, the absence of aspecific part of the structure, or the presence of an unwanted structureon the substrate or on the wafer.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention, where the context allows, is notlimited to optical lithography and may be used in other applications,for example imprint lithography.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The descriptions above are intended to beillustrative, not limiting. Thus it will be apparent to one skilled inthe art that modifications may be made to the invention as describedwithout departing from the scope of the claims set out below.

1.-15. (canceled)
 16. A radiation source arrangement operable to causean interaction between a drive radiation beam and a medium forgeneration of emitted radiation by high harmonic generation, thearrangement comprising: an interaction region positioned at aninteraction plane and configured to receive the medium; a beam blockpositioned upstream of the interaction plane at a beam block plane andconfigured to partially block the drive radiation beam; a beam shaperpositioned upstream of the beam block plane at an object plane andconfigured to control a spatial distribution of the drive radiationbeam; and at least one lens positioned upstream of the interaction planeand downstream of the beam block plane, wherein the lens beingpositioned such that an image of the spatial distribution of the driveradiation beam is formed at the interaction plane.
 17. The arrangementof claim 16, wherein the lens is positioned such that the object planeand the interaction plane are conjugate planes.
 18. The arrangement ofclaim 16, wherein an aperture is positioned downstream of theinteraction plane at an aperture plane and configured to allow at leastpart of the emitted radiation to pass through and to block at least partof the drive radiation beam, wherein the aperture plane is positionedwith respect to the beam block plane and the lens such that an image ofthe beam block is formed at the aperture plane.
 19. The arrangement ofclaim 18, wherein the lens is positioned such that the beam block planeand the aperture plane are conjugate planes.
 20. The arrangement ofclaim 18, wherein a dimension of the beam block in the beam block planerelative to a dimension of the drive radiation beam in the beam blockplane is such that the image of the beam block and the image of thespatial distribution of the drive radiation beam are decoupled.
 21. Thearrangement of claim 20, wherein the dimension of the beam block in thebeam block plane is 30% or less of the dimension of the drive radiationbeam in the beam block plane.
 22. The arrangement of claim 21, whereinthe beam block and the drive radiation beam have substantially circularcross sections in the beam block plane, and wherein the dimensions ofthe beam block and the drive radiation beam are diameters.
 23. Thearrangement of claim 16, wherein a depth of focus of the image of thebeam block does not overlap the interaction plane.
 24. The arrangementof claim 23, wherein a center of the depth of focus of the image of thebeam block is substantially coincident with the aperture plane.
 25. Thearrangement of claim 23, wherein a circle of confusion associated withthe depth of focus of the image of the beam block is larger than theimage of the drive radiation beam at the interaction plane.
 26. Thearrangement of claim 25, wherein the depth of focus of the image of thebeam block has a maximum circle of confusion having a diameter of 35 μmor less.
 27. The arrangement of claim 16, wherein a depth of focus ofthe image of the spatial distribution of the drive radiation beam doesnot overlap the aperture plane.
 28. The arrangement of claim 27, whereina center of the depth of focus of the image of the intensitydistribution of the drive radiation beam is substantially coincidentwith the interaction plane.
 29. The arrangement of claim 23, wherein atleast one of the depth of focus of the image of the beam block and thedepth of focus of the image of the spatial distribution of the driveradiation beam is determined byDepth of focus=2cN(1+m) where c is a maximum circle of confusion, N is aratio of focal length of the lens to a diameter of the drive radiationbeam at the lens, and m is a magnification factor of the image of thebeam block at the aperture plane.
 30. The arrangement of claim 16,wherein the beam shaper comprises a spatial light modulator positionedat the object plane.
 31. The arrangement of claim 30, further comprisinga sensor configured to detect a spatial profile of the drive radiationbeam at the interaction plane; and a feedback controller configured tofeedback data relating to the detected spatial profile of the driveradiation beam to the spatial light modulator, wherein the spatial lightmodulator is configured to control the spatial profile of the driveradiation beam based on the data fed back.
 32. A metrology apparatuscomprising: an arrangement operable to cause an interaction between adrive radiation beam and a medium for generation of emitted radiation byhigh harmonic generation, the arrangement comprising: an interactionregion positioned at an interaction plane and configured to receive themedium; a beam block positioned upstream of the interaction plane at abeam block plane and configured to partially block the drive radiationbeam; a beam shaper positioned upstream of the beam block plane at anobject plane and configured to control a spatial distribution of thedrive radiation beam; and at least one lens positioned upstream of theinteraction plane and downstream of the beam block plane, wherein thelens being positioned such that an image of the spatial distribution ofthe drive radiation beam is formed at the interaction plane.
 33. Alithographic cell comprising the metrology apparatus according to claim32.
 34. A method of causing an interaction between a drive radiationbeam and a medium, for generation of emitted radiation by high harmonicgeneration, the method comprising: blocking the drive radiation beamwith a beam block such that at least part of the drive radiation beam isblocked; propagating the drive radiation beam through at least one lenspositioned downstream of the beam block; impinging the drive radiationbeam on an interaction region comprising the medium and positioneddownstream of the beam block; and focusing the drive radiation at anaperture positioned downstream of the interaction region at an apertureplane such that an image of the beam block is formed at the apertureplane, the aperture being configured to allow at least part of theemitted radiation to pass through and to block at least part of thedrive radiation beam.